Caroline Wozniacki Reveals That She Has Rheumatoid Arthritis

Caroline Wozniacki, her tennis season over after a loss at the WTA Finals on Thursday, revealed that she had recently received a diagnosis of rheumatoid arthritis, an autoimmune disease that can cause joint pain and stiffness and problems with mobility.

“In the beginning, it was a shock,” Wozniacki, 28, said at a news conference in Singapore, the site of the WTA Finals. “Just, you feel like you’re the fittest athlete out there — or that’s in my head, that’s what I’m known for — and all of a sudden you have this to work with.”

But her results in the year’s other major events were not so impressive, and she said she had begun experiencing fatigue and joint soreness after losing in the second round at Wimbledon.

“I thought it was just the flu,” Wozniacki said. “I go to Washington, knees are hurting, my leg is hurting, and I’m like, ‘O.K., well, just move on.’ And I play in Montreal, and something still doesn’t feel right. I wake up, and I can’t lift my arms over my head.”

Wozniacki, who was eliminated from the WTA Finals with a three-set loss to Elina Svitolina in round-robin play, said it had taken time to determine the nature of her illness. She said she had not gotten a definitive answer until after the United States Open, where she lost in the second round to the unseeded Lesia Tsurenko.

“That’s when I really figured it out,” said Wozniacki, who is now ranked No. 3. “I went to see one of the best doctors there is and started treatment.”

There is no known cure for the condition, though patients can go into remission.

“At the end of the day, it’s obviously not ideal for anybody,” Wozniacki said, “and I think when you are a professional athlete, it’s also not even more ideal. But at the end of the day, you find a plan. You figure out what to do. You do your research, and thankfully, there’s great things now you can do to it and do about it.”

Dr. John M. Davis III, a rheumatologist at the Mayo Clinic in Rochester, Minn., provided background on rheumatoid arthritis in a telephone interview. He emphasized that he was speaking in general terms because he was not familiar with Wozniacki’s case.

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Wozniacki warming up before a match at the WTA Finals this week in Singapore.CreditClive Brunskill/Getty Images

“It can occur in children, and the risk goes up with age, but the peak age of onset is mid-50s,” Dr. Davis said. “So 28 is definitely on the young side to be getting it.”

Rheumatoid arthritis is quite different from osteoarthritis, the most common form of arthritis, which is degenerative and typically linked to overuse or prior injury in a joint.

“Inflammatory, or rheumatoid, arthritis is a condition in which the immune system is basically attacking the joints of the body and other tissues,” Dr. Davis said. “So the cause is very, very different, and the treatments are very, very different.”

Though pursuing a career in elite sports with rheumatoid arthritis poses a challenge, Dr. Davis said such a career was conceivable, particularly if the inflammation could be managed effectively.

“I think athletes can still have the hope of regaining or maintaining their physical performance,” he said. “But there are some things to overcome, and it is a matter of time and also a matter of trialing medications. It’s hard to predict exactly how well a patient in general will respond.”

“I would wake up in the morning, and I would literally be scared to open my eyes because I knew how much it was going to hurt,” O’Brien told the Canadian Broadcasting Corporation. “I would have to, like, psych myself up, go: ‘O.K., sit up. You can do this. You just have got to sit up.’”

O’Brien said she had found medication to help manage her symptoms. “I really have very little to no pain on a daily basis, and I really feel like myself again,” she told The Canadian Press.

Venus Williams, 38, has managed to succeed on the women’s tennis circuit after learning in 2011 that she has Sjogren’s Syndrome, an autoimmune disease that can produce some symptoms similar to those associated with rheumatoid arthritis.

Wozniacki said she had been receiving regular treatment and taking medication. This month, she won the China Open, a top event on the women’s tour, and she was also highly competitive before being knocked out of the WTA Finals.

“I’m very proud of how I’ve been so positive through it all,” Wozniacki said. “I didn’t want to talk about it, obviously, during the year, because I don’t want to give anyone the edge or thinking that I’m not feeling well. But I’ve been feeling well. You learn how to just cope after matches. Some days you wake up, and you can’t get out of bed, and you just have to know that’s how it is. But other days, you live and you are fine. You don’t even feel like you have it.”

A version of this article appears in print on , Section B, Page 11 of the New York edition with the headline: Wozniacki Announces Autoimmune Diagnosis. Order Reprints | Today’s Paper | Subscribe